msm: mdp3: Error handling during histogram start failure

The mdp3_session->lock mutex is not unlocked in few error
paths in the histogram start flow. Add changes to unlock
the mutex if histogram start fails for some reason. This
makes sure that post-proc userspace tasks doesn't exit
with mutex lock held thereby avoiding display freeze
issues caused by other display tasks waiting on the
session lock mutex.

Change-Id: Ie16df32b3f4737d5cb064bd0ef30ee3143e9b67b
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>